An understanding of the desaturation process which is involved in the synthesis of polyunsaturated fatty acids such as oleic, linoleic and alpha linolenic acids is essential in order to clarify their roles in membrane systems. We have now under extensive study a soluble stearyl ACP desaturase obtained from plant systems. Since all animal desaturase are membrane bound, we are in a strong position to purify and clarify the mechanism of desaturation using this model system. We are also investigating the synthesis of linoleic and linolenic acids by unique pathways.